Acoustic apparatus



Patented Oct. 8, 1940 UNITED STATES PATENT OFFICE Radio Corporation of Delaware America, a corporation of Application July 23, 1938, Serial No. 220,844

4 Claims.

My invention relates to acoustic apparatus, and

more particularly to high fidelity sound reproducing apparatus including a plurality of loud speakers operated in complementary relation,

each of which reproduces efficiently only a limited band of sound waves in the useful audio frequency spectrum, as for voice and music.

An object of my invention is to improve the fidelity of reproduction in the case of multiple operation of loud speakers operating conjointly over different portions of the audio frequency range.

A further object of my invention is to provide a sound reproducing apparatus, including a plurality of loud speakers operating conjointly over different parts of the audio frequency spectrum, with acoustic structure for, preventing one speaker from deleteriously affecting the performance of another, particularly where the speakers are closely coupled acoustically, as when mounted in a common cabinet enclosure.

In accordance withmy invention, I have found that when a loud speaker, made especially for the eflicient reproduction of higher audio frequency wave band, is mounted on the same bafiie with one adapted to reproduce more emciently a lower frequency wave band, the sound wave pressure generated by the lower frequency speaker, which is appreciable at certain frequencies, causes distortion or objectionable noise in the reproduction from the higher frequency speaker. Where the diaphragm of an extremely high frequency speakerhas been of light weight hardfinish paper. as in the case of Tiedje Patent 2,124,597, dated July 26, 1938, and assigned to the same assignee as the present application, the diaphragm was caused to rattle by the low frequency speaker. In some instances the voice coil is dislocated in the airgap, resulting in striking of the pole pieces, and a quite objectionable noise. In other cases the diaphragm is driven as a Helmholz resonator, as in Olson Patent 1,988,250, an effect which is not desirable in the above combination.

According to my invention, I have overcome the above difficulties by providing an acoustic barrier of simplified construction completely or partially enclosing the rear of the high frequency speaker, for acoustically shielding it from, or for filtering out, certain of the sound pressure waves of the low frequency speaker. In another case, I have isolated the two speakers by enclosing the rear of the low frequency speaker.

For a more complete disclosure of my invention, reference should be had to the following detailed description which should be read in connection with the accompanying drawing, in which,

Figure 1 shows, in a side elevational View, in section, an application of my invention to sound reproducing apparatus;

Figure 2 illustrates a circuit used to operatively connect the loud speakers in my sound reproducing apparatus to a source of audio frequency currents corresponding to voice and music;

Figures 3 and 4 show side elevational frag- 1 mentary views, partly in section, of modifications of the portion of my invention illustrated in Figure 1;

Figure 5 shows, in side elevation, partly in section, a plurality of speakers mounted in a cabinet enclosure in accordance with a modification of my invention;

Figure 6 shows, in side elevation, partly in section, a radio receiving set embodying a still further modification of my invention.

Figure 7 shows, in rear elevation, a view of a modification Figures 5 and 6.

Referring to Figure l, a loud speaker 3, comprising a diaphragm 5 and driving structure 1, which may, for example, be either an electrodynamic or piezoelectric, or equivalent type of actuating unit, is mounted on a baffle 9, that may be of the open type or the front wall of a cabinet, in registering relation with an opening H for the purpose of efficiently reproducing high audio frequency sound Waves. This speaker'may be made, for the purpose of illustration, in accordance with the teachings of the said Tiedje Patent 2,124,597. On the same baifie is mounted a speaker l3, comprising a diaphragm l5 and a driving unit IT, in registering relation with an opening l9 adapted for the more efficient reproduction of low audio frequency sound waves.

For the purpose of preventing any of the undesirable efiects above set forth, which occur when speakers of this type are mounted for operation on a common baffle or equivalent device, an acoustic sound barrier 2, made of fibrous material such as wood, pulp, or of metal, is provided around the rear of the high frequency loud speaker 3, thereby preventing the tendency of" circulation of waves from the low frequency speaker through the opening II in front of the high frequency speaker. In effect, therefore, the baflle opening H is sealed by means of the barrier 2, so far as the low frequency speaker is concerned. The enclosure in 2 is preferably air tight against the baffle and over the speaker, and the natural resonance period is preferably low enough in frequency so as not to deleteriously affect the per-,

formance of the speaker. This novel combination is briefly described in said Tiedje Patent, in which reference is made to my invention. Here the high frequency speaker was disclosed as working in a band of from about 4,000 to 10,000 cycles.

Referring to Figure 2, there is shown a series circuit for connecting the high and low frequency loud speakers, as by means of transformer 24, to the plate circuit of a vacuum tube amplifier in a radio receiving set, phonograph, or the like, represented as a source of audio frequency energy at 23. The high frequency signaling currents flow through voice coil and by-pass condenser 2'3, while the low frequency signaling energy flows mainly through voice coils 25 and 28. Other arrangements for connecting high and low frequency speakers are illustrated in Mlnton et a1. Patent 2,084,160.

In Figure 3 I have illustrated a simplified and inexpensive sound barrier for enclosing the rear of the high frequency speaker of Figure 1, namely, a dished or hemispherical molded or felted enclosure Si, preferably of fibrous material such as soft paper or cardboard, which is cemented at its flanged base to the baffle. Although the material is of relatively light material as compared to wood or metal disclosed in connection with the barrier 2 in Fi ure I, it is highly effective by reason of the rigidity inherent in the shape thereof, for acoustically protecting the speaker from the high pressure sound waves of the low frequency speaker. The interior of the dished barrier may, if desired, be lined with felt or other equivalent sound. deadening material, although such lining is ordinarily not necessary by reason of the nature of material of 3!.

In Figure 4 I have shown a still further simplified and inexpensive sound barrier 33, a species of dished construction made in the form of a paper cone diaphragm of the type used in loud speakers. It may be of the molded or felted type, or more simply of the type which is coned up out of the plane of a blank piece of paper. The inherently strong construction of a cone makes this arrangement effective as a sound barrier, and has the advantage that special machinery is not nec essary to form it, since it can be an ordinary large diaphragm used in standard speakers of large size.

While the barrier 2 in Figur I rather narrowly confines the rear of the speaker acoustically, the fidelity is not substantially reduced thereby in view of the relatively short sound waves involved. Although my invention was originally made in connection with a high frequency speaker operating in a band between 4,000 and 10,000 cycles, as mentioned in said Tiedje Patent, in com bination with a low frequency speaker having an effective upper limit of around 4,000 cycles, the trend has subsequently been toward a combination of speakers having a much lower transition point between the bands, say around 300 or 400 cycles. In other words, the low frequency speakers have been designed to produce quite effectively the very low audio frequencies, while the higher frequency speakers have been made to be more effective between this low transition frequency and some upper limit of the audio range empl o yed. In some cases, it is preferable to split up the range into three bands, adapted to be covered by three speakers, bands which may be referred to as low, intermediate and high. In all such cases, my invention has particular utility. In speaking of high and low frequency speakers,

therefore, I use these terms in a relative sense. The high frequency band or speaker may refer to an intermediate band of frequencies that is high with respect to the low frequency band; or the intermediate band may be considered as the low frequency band. with respect to the extreme high frequency band.

With the foregoing in mind, referring to Figure 5, I have found it desirable in some cases, to employ three speakers, 3, 4 and i4, splitting the range up into low, intermediate and high frequency bands, as above mentioned. While completely enclosing the high frequency speaker 3 at the rear is practical, I find it desirable to provide an opening as shown at 18 in the rear wall of a barrier 2, preferably of sheet metal, for a speaker 4 designed to cover an intermediate frequency band for relieving the sound pressure, particularly at the low end of the band. For the purpose of impeding or damping the sound waves from the lower frequency speaker tending to enter through this opening, I have provided a covering of acoustic resistance material 20, preferably of thin metal gauze made in accordance with the teachings of Scheldorf Patent 2,065,751. A layer of felt 22 may also be provided over the inside of the barrier 26:, for the purpose of preventing high frequency sound wave reflection between the walls and the diaphragm with consequent tendency toward distortion. This felt also has acoustic resistance properties over the opening [8, and may supplement the action of the gauze 20, or may well be used in place thereof. The damping action. is also beneficial with respect to low frequency resonance within the barrier enclosure set up by the speaker. It is desirable that the natural period of the enclosure be sufficiently low as not to interfere with the performance of the speaker, but in case it is not so, the acoustic resistance material and/or felt lining is sufliciently efiective to overcome this difiiculty.

In Figure 5 I have shown the three speakers 3, 4 and i4, mounted inside of a box-like enclosure, which may represent the cabinet of a radio receiver or phonograph, with or without a removable rear wall. In this arrangement, particularly when used within a complete enclosure, the speakers are closely coupled acoustically by means of the cabinet enclosure common to all, and without the use of my barrier means the low frequency speaker would seriously affect the performance of the others. In this modification, speaker i4 covers efiiciently a band from about 300 or 400 cycles downward while speaker 4 covers a band from 400 to 4,000 and speaker 3 from about 4,000 to 10,000 or 14,000 cycles. The rear of the high frequency speaker is closed by the barrier 33 shown in Figure 4 although other suitable forms, e. g., those of Figures 1 and 3, may be used. The barrier of Figure 3 may be designed for use with speaker 4, if desirable. If desired the lowest frequency speaker l4 may be provided with a barrier like that for speaker 4, having acoustic resistance material, instead of using one for speaker 4. See Figure '7.

In Figure 6 I have shown amodification of my invention as applied to a radio receiver wherein the rear of the low frequency speaker I4 is substantially isolated from the rear of a higher frequency speaker 6 by means of a curved wall 4| sealed to the cabinet walls. This curved wall is preferably made of plywood and, although thin, is inherently rigid by reason of its shape. One or more of the walls may, if desired, be covered by a layer of felt 32, as illustrated in connection with the curved wall. In this modification, the low frequency speaker l4 being completely enclosed at its rear, it is not necessary to enclose the rear of the higher frequency speaker 6. A plurality of tubular openings 42 are provided to improve fidelity in accordance with the teachings in Thuras Patent 1,869,178. In effect, this arrangement consists of a common battle and boxlike rear enclosure with a partition subdividing the enclosure. With the tubular openings 42, having low pass filter characteristics, the desirable effects of an open bafile is obtained while at the same time the effect upon the high frequency speaker, that would ordinarily occur, is substantially prevented. Here the barrier prevents interference with speaker 6 while the cooperation of tubes 42 overcomes the disadvantage of enclosing the rear of a low frequency speaker, serving to bring out the low frequency waves from the rear to the front in proper phase relation.

While in Figure 6 I have shown but two speakers, l4 being adapted to cover a band from around 400 cycles downward to, say, 30 cycles, and speaker 6 being designed for a band from around 400 upwards, three speakers obviously may be used to cover the range, as shown in Figure '7. In such event it is not necessary to protect the rear of the intermediate speaker 4, it being left open to the atmosphere or to the cabinet enclosure, as shown. The high frequency speaker 3 may, however, be protected with a barrier 33, from the intermediate speaker in such case. Where only two speakers are used to cover the. entire range, as in Figure 6, it may be desirable to make at least one of these into a compound speaker, as of the type disclosed in Ringel Patent 2,007,747.

While I have shown the low frequency speaker I4 with a barrier 4|, as in Figure 6, I have provided a series of openings I8 in the lower portion of 4| whereby the action of the felt lining 32 acts as acoustic resistance for damping resonance of the enclosure.

I have therefore. provided an arrangement wherein sound waves may be reproduced with unusual fidelity, and while I have shown several specific examples, it will be obvious to those skilled in the art that many other modifications embodying the principles disclosed may readily be employed. Where I have disclosed a plurality of speakers mounted on a common baflle of the flat type, it will be obvious to those skilled in the art that different types of bailles may be employed, for example, a type having directional horn characteristics such as shown in Haigis Patent 1,849,486.

I intend, therefore, to be limited only as indicated by the scope of the following claims.

What I claim is:

1. In a sound reproducing device, a bafile having a plurality of sound openings, a plurality of closely coupled loud speakers mounted thereon in operative relation with respect to said open ings, respectively, one of said speakers being adapted to reproduce most eificiently low audio frequency sound waves, and another of said speakers having a relatively light weight diaphragm adapted to reproduce relatively higher audio frequency waves most efficiently, and acoustic barrier means covering the opening to the rear of said speaker having the relatively light weight diaphragm for substantially preventing the sound pressure from said low frequency speaker from deleteriously affecting the performance of the said higher frequency speaker.

2. In a wide range sound reproducing system, a high frequency loud speaker and a low frequency loud speaker mounted in closely coupled acoustic relation in a common housing and arranged to reproduce more efficiently the higher and lower audio frequency portions of said range, respectively, said high frequency speaker being disposed in a high pressure sound wave region of said low frequency speaker, and acoustic barrier means forming an enclosure of the space to the rear of said high frequency speaker for protecting said high frequency speaker from said. 

